CN112334384B

CN112334384B - Propulsion unit and aircraft comprising same

Info

Publication number: CN112334384B
Application number: CN201980039932.8A
Authority: CN
Inventors: A.普拉格
Original assignee: Wing Aviation LLC
Current assignee: Wing Aviation LLC
Priority date: 2018-06-13
Filing date: 2019-06-10
Publication date: 2021-10-08
Anticipated expiration: 2039-06-10
Also published as: CN112334384A; SG11202012053TA; AU2019284351A1; EP3797070A1; AU2019284351B2; EP3797070B1; US10843795B2; WO2019241110A1; US20190382106A1; EP3797070A4

Abstract

A propulsion unit includes an electric motor rotor, propeller blades, and a pivot stop. The motor rotor rotates about a central axis of rotation. The propeller blades include a first propeller blade and a second propeller blade, each propeller blade having a proximal base mounted to the motor rotor such that the propeller blades are rotatable about a central axis of rotation. The second propeller blades are pivotally attached to the motor rotor to pivot about a central axis of rotation for a limited angle independent of the motor rotor. The pivot stop mechanically limits the amount of pivoting of the second propeller blade relative to the first propeller blade.

Description

Propulsion unit and aircraft comprising same

Technical Field

The present disclosure relates generally to propeller blade mounts, and particularly, but not exclusively, to propeller blade mounts for unmanned aerial vehicles.

Background

Unmanned vehicles, which may also be referred to as autonomous vehicles (vehicles), are vehicles that are capable of traveling without the actual presence of a human operator. The unmanned vehicle may operate in a remote control mode, an autonomous mode, or a partially autonomous mode.

When the unmanned vehicle is operating in a remote control mode, a pilot or driver at a remote location may control the unmanned vehicle by commands sent to the unmanned vehicle via a wireless link. When unmanned vehicles operate in an autonomous mode, unmanned vehicles typically move based on preprogrammed navigation waypoints, dynamic automation systems, or a combination of these. Further, some unmanned vehicles may operate in both a remote control mode and an autonomous mode, and in some cases may be simultaneously. For example, a remote pilot or pilot may wish to hand navigation to an autonomous system when another task is performed manually (such as operating a mechanical system for picking up objects), as an example.

There are various types of unmanned vehicles for various environments. For example, unmanned vehicles exist for operation in air, on the ground, under water, and in space. In general, Unmanned Aerial Vehicles (UAVs) or drones are becoming increasingly popular. As their designs have perfected and their functions have expanded, their suitability for commercial use is expected to increase. Designs that improve the efficiency and durability of UAVs will expand their mission capabilities.

Disclosure of Invention

Drawings

Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. Not all instances of elements are necessarily labeled to avoid obscuring the drawings in place. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles described.

Fig. 1 is a perspective view of an aircraft with propeller blades of a vertical propulsion unit in a deployed position according to an embodiment of the present disclosure.

Fig. 2 is a plan view of an aircraft with the propeller blades of the vertical propulsion unit in a stowed position according to an embodiment of the present disclosure.

Fig. 3 is a perspective view of a propulsion unit according to an embodiment of the present disclosure with the propeller blades in a deployed position.

Fig. 4A is a perspective view of a propulsion unit according to an embodiment of the present disclosure with the propeller blades in a stowed position.

Fig. 4B is a plan view of a propulsion unit according to an embodiment of the present disclosure with the propeller blades in a stowed position.

Fig. 4C is a side view of a propulsion unit according to an embodiment of the present disclosure with the propeller blades in a stowed position.

Fig. 5A is an exploded view of a propulsion unit having folded propeller blades according to an embodiment of the present disclosure.

Fig. 5B is a perspective view of a base insert spacer including a washer flange, alignment boss, and pivot stop according to an embodiment of the present disclosure.

Fig. 6 is a cross-sectional view of a propulsion unit having folded propeller blades according to an embodiment of the present disclosure.

Detailed Description

Embodiments of systems, apparatus, and methods of operation for folding propeller blades to reduce aerodynamic drag are described herein. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

The embodiments disclosed herein describe mechanical structures and techniques for pivotally mounting propeller blades to an electric motor rotor such that the propeller blades can be passively folded from a deployed position when in use to a stowed position when idle. The stowed position provides reduced drag during forward flight. In one embodiment, the folding of the propeller blades into the stowed position is achieved passively by wind resistance, while the unfolding back into the deployed position is achieved by a pivot stop which engages the propeller blades into a rotationally offset position relative to each other. In another embodiment, inertial and motor braking may also be used to assist in passive folding or active activated folding of the propeller blades into the stowed position.

Although the propeller blade mounting structure may be used in various vehicle applications, the described techniques are particularly useful in applications having separate propulsion units for horizontal and vertical propulsion. In particular, the described technology is well suited for vertical takeoff and landing aircraft.

Fig. 1 and 2 illustrate an aircraft 100 according to an embodiment of the present disclosure. The illustrated embodiment of the aircraft 100 is a vertical takeoff and landing (VTOL) Unmanned Aerial Vehicle (UAV) that includes

separate propulsion units

106 and 112 for providing horizontal and vertical propulsion, respectively. The aircraft 100 is a fixed wing aircraft, as the name implies, having a wing assembly 102, which wing assembly 102 may generate lift based on the wing shape and forward airspeed of the aircraft when propelled horizontally by a propulsion unit 106. Fig. 1 is a perspective view of an aircraft 100 operating during vertical takeoff or landing with the propeller blades of a vertical propulsion unit 112 deployed to provide vertical propulsion. Fig. 2 is a plan view of the aircraft 100 operating in a horizontal cruise mode, with the propeller blades of the vertical propulsion units 112 idle (i.e., not rotating) and stowed to reduce the drag profile during forward motion. As shown, the propeller blades of the vertical propulsion unit 112 are stowed and passively aligned to reduce drag due to wind resistance caused by forward motion of the aircraft 100. Instead, the propeller blades of the vertical propulsion unit 112 are deployed in fig. 1 due to the engagement of the pivot stops when the vertical propulsion unit 112 rotates.

The illustrated embodiment of the aircraft 100 has an airframe that includes a wing assembly 102, a fuselage 104, and a boom assembly 110. In one embodiment, the airframe 104 is modular and includes a battery module, an avionics module, and a mission payload module. The modules may be separate from each other and may be mechanically secured to each other to continuously form at least a portion of the fuselage or body.

The battery module includes a cavity for receiving one or more batteries for powering the aircraft 100. The avionics module houses the flight control circuitry of the aircraft 100, which may include a processor and memory, communication electronics and antennas (e.g., cellular transceiver, wifi transceiver, etc.), and various sensors (e.g., global positioning sensor, Inertial Measurement Unit (IMU), magnetic compass, etc.). The mission payload module houses devices associated with the mission of aircraft 100. For example, the task payload module may include a payload actuator for holding and releasing an externally attached payload. In another embodiment, the task payload module may include a camera/sensor device holder for carrying a camera/sensor device (e.g., camera, lens, radar, lidar, pollution monitoring sensor, weather monitoring sensor, etc.).

As shown, the aircraft 100 includes horizontal propulsion units 106 positioned on the wing assemblies 102, which may each include a motor, a motor rotor having a shaft, and propeller blades, for propelling the aircraft 100 horizontally. The illustrated embodiment of the aircraft 100 also includes two boom assemblies 110 secured to the wing assembly 102. A vertical propulsion unit 112 is mounted to the boom assembly 110. The vertical propulsion units 112 may also each include a motor, a motor rotor having a shaft, and propeller blades for providing vertical propulsion. As described above, vertical propulsion units 112 may be used during hover modes where aircraft 100 is descending (e.g., arriving at a delivery location), ascending (e.g., after delivery), or maintaining a constant altitude. A stabilizer 108 (or tail) may be included in the aircraft 100 to control pitch and stabilize the yaw (left or right turn) of the aircraft during cruise. In some embodiments, during cruise mode, vertical propulsion units 112 are disabled, while during hover mode, horizontal propulsion units 106 are disabled. In other embodiments, vertical propulsion units 112 are only low power during cruise mode, and/or horizontal propulsion units 106 are only low power during hover mode.

During flight, the aircraft 100 may control the direction and/or speed of its motion by controlling its pitch, roll, yaw, and/or altitude. Thrust from the horizontal propulsion unit 106 is used to control airspeed. For example, the stabilizer 108 may include one or more rudders 108a for controlling the yaw (and pitch due to the V-tail configuration shown) of the aircraft, and the wing assembly 102 may include ailerons 102a for controlling the roll of the aircraft. In other embodiments, the aircraft 100 may include elevators for independently controlling pitch. As another example, increasing or decreasing the speed of all propeller blades simultaneously may cause the aircraft 100 to increase or decrease its altitude, respectively.

Many variations to the fixed-wing aircraft shown are possible. For example, aircraft having more wings (e.g., an "x-wing" configuration having four wings) are also possible. Although fig. 1 and 2 illustrate one wing assembly 102, two boom assemblies 110, two horizontal propulsion units 106, and six vertical propulsion units 112 per boom assembly 110, it should be understood that other variations of the aircraft 100 may be implemented with more or fewer of these components. Furthermore, although fig. 1 and 2 show the propulsion units (both vertical and horizontal propulsion units) as each comprising four propeller blades, in various embodiments the propulsion units may comprise two, three or more than four propeller blades.

Fig. 3, 4A, 4B, 4C, 5A, 5B, and 6 are different views illustrating various aspects of a propulsion unit 300 (e.g., vertical propulsion unit 112) according to an embodiment of the present disclosure. Fig. 3 is a top perspective view of the propulsion unit 300 with propeller blades in a deployed position, fig. 4A-4C are various views of the propulsion unit 300 with propeller blades in a stowed position, fig. 5A is an exploded view of the propulsion unit 300, and fig. 6 is a cross-sectional view of the propulsion unit 300 along line a-a' in fig. 4B.

The illustrated embodiment of the propulsion unit 300 includes a motor base 305, propeller blades 310A-310D (collectively 310), insert spacers 515A-515C (collectively 515), a stationary cover 520, and mechanical fasteners 525. The illustrated embodiment of the motor base 305 includes a motor rotor, a stator 637, and a motor mount 340. The illustrated embodiment of the motor rotor includes a rotor shaft 632, a protruding ridge 534, and a rotor bell 335. The illustrated embodiments of the propeller blades 310 each include a proximal base 545 and a distal tip 555. The proximal bases 545 each include a cutout 550 and an aperture 551. The illustrated embodiment of the insert spacer 515A includes pivot stops 560A and 560B disposed on a washer flange 565, the washer flange 565 having a hole 570, an alignment tab 575, and a recess 580. The illustrated embodiments of the insert spacers 515B-515C each include a

pivot stop

560A and 560C and a washer flange 565 with an aperture 585. It should be understood that not all instances of elements have been labeled so as not to unduly obscure the drawings.

As shown in fig. 3 and 4A, the propeller blades 310 are mounted to the motor base 305 in a manner that allows them to pivot from a deployed, deployed position (fig. 3) to a stowed, folded position (fig. 4A). The transition from unrolling to folding or from folding to unrolling may be a passively controlled transition or actively initiated. As the motor rotor rotates, the propeller blades 310 unfold (spread apart) and the inertia/drag of the propeller blades causes the pivot stops 560A and 560C to engage the cutouts 550 in the proximal base 545 of the propeller blades 310. This engagement rotationally accelerates the propeller blades 310 to the deployed position. In one embodiment, the bottommost propeller blade 310D is non-pivotally attached to the motor rotor for rotation about the central axis of rotation 301 in a fixed relationship therewith. Instead, the propeller blades 310A-310C are pivotally attached to the motor rotor to pivot relative to each other about the central axis of rotation 301 by a limited angle independent of the motor rotor.

Referring to fig. 5A, pivot stops 560A and 560C engage against the ends of the cut-outs 550 to rotate the propeller blades 310A-310C but also slide within the cut-outs 550 to allow a limited amount of pivoting relative to each other and relative to the motor rotor to allow folding to a stowed position. When the motor rotor stops rotating, the wind drag due to the forward motion of the aircraft 100 causes the propeller blades 310A-310D to pivot to a smaller cross-sectional, lower drag stowed position. Thus, the stowed position reduces the drag profile of the propulsion unit 300 in a direction perpendicular to the central axis of rotation 301. Fig. 4A, 4B and 4C show different views of such a reduced resistance profile.

Fig. 5A and 5B show details of the pivot stops that facilitate limited pivoting of the propeller blades 310A-310C. As shown, the pivot stop 560 is a protrusion integrated into the insert spacer 515. In other embodiments, the pivot stop 560 may be a post, fastener, or another component attached to the insert spacer 515. An insert spacer 515 is inserted between each propeller blade 310. In the illustrated embodiment, the bottom-most insert spacer 515A includes an alignment boss 575 that is inserted through the holes 551 of the propeller blades 310A-310C and the holes 585 of the

other insert spacers

515B and 515C. The alignment tabs 575 serve a variety of purposes. First, the alignment tabs 575 position and align the upper propeller blades 310A-310C and the insert spacers 515B-515C for rotation about the central axis of rotation 301. Second, the alignment tabs 575 serve as rotational bearings for the upper propeller blades 310A-310C. Third, the alignment tabs 575 are fixed-length offsets to which the stationary cover 520 is clamped by mechanical fasteners 525. The alignment tabs 575 are designed to have a suitable length for gripping the proximal bases 545 of the propeller blades 310A-310C to limit excessive pitch or dihedral bending of the propeller blades 310A-310C while allowing the propeller blades 310A-310C to pivot relative to one another. In practice, the length of the alignment tabs 575 should be selected such that the propeller blades 310A-310C will pivot to the stowed position only under the influence of wind resistance at typical cruise speeds of the aircraft 100 when the propulsion unit 300 is not rotating.

In one embodiment, the folding of the propeller blades 310 may be actively triggered/initiated by a short reverse torque pulse of the motor. These motor torque pulses may cause the motor to reverse briefly (or only slow down) (e.g., less than one revolution) to initiate folding, which is passively aligned by windage. Thus, blade inertia may be used to help actively initiate folding to the stowed position. In some embodiments, a small propeller blade diameter may require a reverse torque pulse to initiate folding due to limited wind resistance on the propeller blade itself.

The insert spacers 515 each include a washer flange 565 having a widened surface that helps control the dihedral bending of the propeller blades 310 and serves to offset the propeller blades 310 relative to each other along the central axis of rotation 301 by a fixed offset. This "vertical" offset of each propeller blade 310 helps to fold the propeller blades 310 into the stowed position without catching aerodynamic surfaces (e.g., leading or trailing edges of the propeller blades, etc.) from one another. In one embodiment, the vertical offset provided by the washer flange 565 is minimized. In one embodiment, the washer flange 565 is integrated into the proximal base 545 of the propeller blade 310. This vertical mounting offset also facilitates mounting the holes 551 of the propeller blades 310 concentrically along the central axis of rotation 301 such that all the propeller blades 310 rotate about a common axis (i.e., the central axis of rotation 301). The concentric mounting of the propeller blades 310 lends itself to a reduced cross-sectional profile when the propeller blades 310 are stowed.

In the illustrated embodiment, the lower propeller blades 310D are mounted in fixed relation to the motor rotor. In other words, the propeller blades 310D do not have a limited amount of pivoting relative to the motor rotor. In one embodiment, the propeller blade 310D is the only propeller blade that does not pivot independently of the motor rotor. Thus, when the propeller blades 310D are folded to the stowed position under the influence of wind resistance, the propeller blades 310D also rotate the motor rotor. In the illustrated embodiment, the insert spacer 515A securely clamps the proximal base 545 of the propeller blade 310D to the motor rotor under the abutment pressure transferred from the mechanical fastener 525 by the alignment tabs 575. In the illustrated embodiment, the pivot stop 560B is larger than the other pivot stops 560C. In one embodiment, the pivot stop 560B is sized to mate with the cutout 550 on the propeller blade 310D so as not to allow any pivoting relative to the motor rotor. Rather, the pivot stop 560B acts as a ledge (ridge) to balance the pressure from the mechanical fastener 525.

The insert spacer 515A also includes a recess 580 in the bottom side of the washer flange 565. The recess 580 is sized to mate with the protruding ridge 534 on the motor rotor. The recess 580 is an alignment feature that centers the upper components (the propeller blades 310 and the insert spacer 515) on the central rotational axis 301 of the motor rotor. In one embodiment, the protruding ridge 534 is the butt end of the rotor shaft 632. In other embodiments, the protruding ridge 534 may be part of the rotor bell 335.

The pivot stops 560 distribute the unrolling of the propeller blades 310 at equal rotational separation angles when fully deployed. Thus, in the illustrated embodiment of the propulsion unit 300 having four propeller blades 310, the center-to-center rotational offset between the propeller blades 310 when fully deployed is 90 degrees. However, the propeller mounting techniques described herein are equally applicable to propulsion units having two, three or more propeller blades. In a dual propeller configuration, the pivot stops 560 distribute the propeller blades by 180 degrees. In the triple propeller configuration, the pivot stops 560 distribute the propeller blades by 120 degrees. Of course, the number of insert spacers 515 will vary based on the number of propeller blades.

In the illustrated embodiment, the pivot stop 560 is integrated into the insertion spacer 515. This design allows for the use of multiple instances of a single common propeller blade 310. However, in other designs, the pivot stop 506 may be integrated into the proximal base 545 of each propeller blade 310. Alternatively, a single pivot stop 560 may be integrated into the rotor bell 335, and the ends of each cutout 550 are staggered for the propeller blades 310.

The above description of illustrated embodiments of the invention, including what is described in the abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.

These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.

Claims

1. A propulsion unit, comprising:

a motor rotor rotating about a central axis of rotation;

propeller blades including first and second propeller blades, each propeller blade having a proximal base mounted to the motor rotor such that the propeller blades are rotatable about the central axis of rotation, wherein the second propeller blade is pivotally attached to the motor rotor to pivot about the central axis of rotation a limited angle independent of the motor rotor;

a pivot stop mechanically limiting the amount of pivoting of the second propeller blade relative to the first propeller blade; and

a washer flange disposed between the proximal base of the first propeller blade and the proximal base of the second propeller blade to offset the second propeller blade relative to the first propeller blade along the central axis of rotation.

2. The propulsion unit according to claim 1, wherein the propeller blades fold or unfold between a deployed position providing propulsion and a stowed position having a reduced resistance profile in a direction perpendicular to the central axis of rotation.

3. The propulsion unit according to claim 1, wherein the first propeller blade is non-pivotally attached to the motor rotor to rotate about the central axis of rotation in a fixed relationship with the motor rotor.

4. The propulsion unit of claim 1, wherein the pivot stop includes a pair of rotationally offset protrusions extending from opposite sides of the washer flange, the rotationally offset protrusions engaging cutouts in the proximal bases of the first and second propeller blades.

5. The propulsion unit of claim 1, further comprising:

an alignment boss attached to and extending from the gasket flange, the bore in the proximal base of the second propeller blade being positioned with respect to the alignment boss.

6. The propulsion unit of claim 5, wherein the washer flange includes a recess that mates with and aligns with a protruding ridge on the motor rotor.

7. The propulsion unit of claim 5, further comprising:

a stationary cover for clamping the propeller blades and the washer flange to the motor rotor; and

a mechanical fastener inserted through a hole in the stationary cover and a hole in the alignment boss to screw into the motor rotor.

8. A propulsion unit, comprising:

a motor rotor rotating about a central axis of rotation;

propeller blades including first, second and third propeller blades, each propeller blade having a proximal base mounted to the motor rotor such that the propeller blades are rotatable about the central axis of rotation, wherein the second propeller blade is pivotally attached to the motor rotor to pivot about the central axis of rotation a limited angle independent of the motor rotor;

a first pivot stop mechanically limiting the amount of pivoting of the second propeller blade relative to the first propeller blade; and

a second pivot stop integrated onto a first washer flange, the first washer flange being disposed to offset the proximal base of the second propeller blade and the proximal base of the third propeller blade.

9. The propulsion unit according to claim 8, wherein the first and second pivot stops offset the rotational position of the propeller blades by equal angles relative to each other when the propeller blades are deployed to a deployed position.

10. The propulsion unit of claim 8, wherein the propeller blades further comprise fourth propeller blades, the propulsion unit further comprising:

a third pivot stop integrated onto a second washer flange disposed between the proximal base of the third propeller blade and the proximal base of the fourth propeller blade.

11. An aircraft, comprising:

a body;

a horizontal propulsion unit mounted to the airframe and oriented to provide horizontal propulsion to the aircraft; and

a vertical propulsion unit mounted to the airframe and oriented to provide vertical propulsion to the aircraft, the vertical propulsion unit comprising:

a motor rotor rotating about a central axis of rotation;

a first pivot stop disposed relative to the first propeller blade and the second propeller blade to mechanically limit the amount of pivoting of the second propeller blade relative to the first propeller blade; and

a first washer flange disposed between said proximal base of said first propeller blade and said proximal base of said second propeller blade to offset said second propeller blade relative to said first propeller blade along said central axis of rotation.

12. The aerial vehicle of claim 11 wherein during forward movement of the aerial vehicle, the propeller blades of the vertical propulsion unit fold or unfold between a deployed position providing the vertical propulsion and a stowed position having a reduced drag profile.

13. The aerial vehicle of claim 12 wherein said propeller blades are deployed to said deployed position by engaging said first pivot stop between said first propeller blade and said second propeller blade when said electric motor rotor of said vertical propulsion unit is rotating, said propeller blades being folded to said stowed position due to windage caused by forward movement of said aerial vehicle when said electric motor rotor of said vertical propulsion unit is not rotating.

14. The aircraft of claim 12 wherein the vertical propulsion unit is adapted to apply a reverse torque pulse to initiate folding of the propeller blades into the stowed position.

15. The aerial vehicle of claim 11 wherein the first propeller blade is non-pivotally attached to the motor rotor to rotate about the central axis of rotation in a fixed relationship with the motor rotor.

16. The aerial vehicle of claim 11 wherein the first pivot stop comprises a pair of rotationally offset protrusions extending from opposite sides of the first washer flange, the rotationally offset protrusions engaging cutouts in the proximal bases of the first and second propeller blades.

17. The aircraft of claim 11, further comprising:

an alignment boss attached to and extending from the first washer flange, the bore in the proximal base of the second propeller blade being positioned with respect to the alignment boss.

18. The aircraft of claim 17, wherein the first washer flange includes a depression that mates with and aligns with a protruding ridge on the electric machine rotor.

19. The aerial vehicle of claim 11 wherein the propeller blades further comprise a third propeller blade, the aerial vehicle further comprising:

a second pivot stop integrated onto a second washer flange disposed between the proximal base of the second propeller blade and the proximal base of the third propeller blade.

20. The aerial vehicle of claim 19 wherein the first and second pivot stops offset the rotational positions of the propeller blades by equal angles relative to each other when the propeller blades are deployed to a deployed position.

21. An aircraft, comprising:

a body;

a motor rotor rotating about a central axis of rotation;

propeller blades including first and second propeller blades, each propeller blade having a proximal base mounted to the motor rotor such that the propeller blades are rotatable about the central axis of rotation, wherein the second propeller blade is pivotally attached to the motor rotor to pivot about the central axis of rotation a limited angle independent of the motor rotor; and

a pivot stop disposed relative to the first and second propeller blades to mechanically limit the amount of pivoting of the second propeller blade relative to the first propeller blade, wherein during forward movement of the aircraft, the propeller blades of the vertical propulsion unit fold or unfold between a deployed position providing the vertical propulsion and a stowed position having a reduced drag profile,

wherein the vertical propulsion unit is adapted to apply a reverse torque pulse to initiate folding of the propeller blades into the stowed position.

22. A propulsion unit, comprising:

a motor rotor rotating about a central axis of rotation;

a pivot stop configured to mechanically limit the amount of pivoting of the second propeller blade relative to the first propeller blade by engaging with cutouts in the proximal base of the first propeller blade and the proximal base of the second propeller blade.

23. The propulsion unit of claim 22, wherein the pivot stop comprises a pair of rotationally offset protrusions that engage the cutouts in the proximal base of the first propeller blade and the proximal base of the second propeller blade.